Open Data supplied by Natural Environment Research Council (NERC)

Niskin Bottle

The Niskin bottle is a device used by oceanographers to collect subsurface seawater samples. It is a plastic bottle with caps and rubber seals at each end and is deployed with the caps held open, allowing free-flushing of the bottle as it moves through the water column.

Standard Niskin

The standard version of the bottle includes a plastic-coated metal spring or elastic cord running through the interior of the bottle that joins the two caps, and the caps are held open against the spring by plastic lanyards. When the bottle reaches the desired depth the lanyards are released by a pressure-actuated switch, command signal or messenger weight and the caps are forced shut and sealed, trapping the seawater sample.

Lever Action Niskin

The Lever Action Niskin Bottle differs from the standard version, in that the caps are held open during deployment by externally mounted stainless steel springs rather than an internal spring or cord. Lever Action Niskins are recommended for applications where a completely clear sample chamber is critical or for use in deep cold water.

Clean Sampling

A modified version of the standard Niskin bottle has been developed for clean sampling. This is teflon-coated and uses a latex cord to close the caps rather than a metal spring. The clean version of the Levered Action Niskin bottle is also teflon-coated and uses epoxy covered springs in place of the stainless steel springs. These bottles are specifically designed to minimise metal contamination when sampling trace metals.

Deployment

Bottles may be deployed singly clamped to a wire or in groups of up to 48 on a rosette. Standard bottles have a capacity between 1.7 and 30 L, while Lever Action bottles have a capacity between 1.7 and 12 L. Reversing thermometers may be attached to a spring-loaded disk that rotates through 180° on bottle closure.

Dissolved nutrient data series for cruise Charles Darwin CD132

Content of Data Series

Parameter

Unit

Parameter Code

Number of stations

Comments

Ammonium

µmol/l

AMONAATX

322

None

Ammonium (nanomolar system)

µmol/l

AMONNATX

71

None

Nitrate+Nitrite (LWCC)

µmol/l

NTRZLWTX

50

None

Nitrate+Nitrite

µmol/l

NTRZAATX

567

None

Nitrite

µmol/l

NTRIAATX

558

None

Phosphate

µmol/l

PHOSAATX

561

None

Silicate

µmol/l

SLCAAATX

556

None

Originator's Protocol for Data Acquisition and Analysis

The following methodology was compiled from information extracted from the CD132 cruise report (Burkill 2002) and from information provided by the originator.

Samples for nutrient analyses were collected from water sampled using the CTD rosette system equipped with 24x20L Niskin sampling bottles. Samples were collected at 36 CTD stations including pre-dawn casts (biogeochemistry, primary production, and nitrogen uptake and regeneration experiments) generally down to 300m, deep casts down to 2500m and mid-day high vertical resolution sampling casts.

Water samples were sub-sampled into acid-cleaned 60 ml HDPE (Nalgene) bottles. Analysis for the nutrient samples was in every case complete within 3 hours of sampling. Clean handling techniques were employed to avoid any contamination of the samples, particularly by ammonium. Samples were analysed without pre-filtration. No samples were stored.

The nutrient analyser was the five-channel Technicon AAII segmented flow autoanalyser. The chemical methodologies used were according to Brewer and Riley (1965) for nitrate, Grasshoff (1976) for nitrite, Kirkwood (1989) for phosphate and silicate, and Mantoura and Woodward (1983) for ammonium.

The nanomolar ammonium system is an adaptation from Jones (1991) which uses a fluorescence analysis technique following ammonia gas diffusion out of the samples, passing across a hydrophobic teflon membrane due to pH differential chemistry.

All CTD samples were analysed with a negligible sample loss rate. The ageing Technicon 5-channel system showed its reliability and reproducibility in the extreme environment of tropical, on-board deployment.

The ammonium fluorescence system performed well early in the cruise before developing a severe loss of sensitivity, which was diagnosed as a fluorometer problem. The system was inoperable for the second half of the cruise and the colorimetric system using the Technicon auto-analyser was used for the remainder of the cruise with good performance.

This cruise was the first deployment of a new unique multi-channel nanomolar analyser combining the segmented flow colorimetric analytical techniques with a Liquid Waveguide Capillary Cell (LWCC). The system is still essentially a developmental analyser. Of the two channels available during the cruise, one was used for the analysis of nitrate and nitrite while the other channel was used to develop the phosphate system into an operational unit. The nitrate channel operated successfully at a sensitivity of about 1 nanomole or better and performed well for the majority of the cruise. Due to lack of time and modern high-precision equipment, it was not possible to make the phosphate channel operate to any sort of acceptable level.

BODC Data Processing Procedures

Data were submitted to BODC in an Excel spreadsheet. Sample metadata were checked against information held in the database and any discrepancy was checked with the data originator. Parameter codes defined in BODC parameter dictionary were assigned to the variables. Data were then loaded into BODC's Research Project Database under an Oracle Relational Database Management System without modification except for the following:

For CTD132_20, two samples were taken at 60m. This was not a depth recording error (confirmed by originator) and a mean of the pair of values was loaded into the BODC database.

For Nitrate+Nitrite measured by LWCC (parameter code NTRZLWTX), values were converted from nanomoles per litre to micromoles per litre.

A record was not loaded for CTD132_006 at 5m - this depth is in the submitted data but not the Seabird profile, as the bottle did not close at this depth. The record was removed on the advice of the originator.

For CTD132_024 the original file shows two samples at 25m. The Seabird profile showed one sample at 25m and one at 35m. This was corrected after advice from originator that it was a typing error.

For CTD132_001, the original file wrongly shows two samples at 10m. The depth profile was adjusted to conform to the Seabird profile after confirmation from the originator.

In the original data file, the column heading "Nitrate = [NO3]: nanomolar" refers to the measurement of NO3 plus NO2 by LWCC (confirmed by originator).

The data in the column headed "[NO3]" in the original file has not been included as this was derived from the subtraction of the "[NO2-]" column values from the "[NO3]+[NO2]" values.

For NH4 measured by the ammonium nanoanalyser, no conversion of units was performed as the data appeared to be presented in micromoles/litre.

Data Quality Report

Although the quality of the data is not affected by what follows, users should be aware that:

Please note:the supplied parameters may not have been sampled from all the bottle firings described in the table above. Cross-match the Sample Reference Number above against the SAMPRFNM value in the data file to identify the relevant metadata.